This invention relates generally to optical networks.
A planar light circuit is an optical circuit that uses integrated waveguides. These waveguides may be integrated into a substrate that, in some embodiments, may be an integrated circuit substrate. The planar light circuit may be formed using techniques that are known in forming integrated circuits.
Commonly it is desired to monitor the power in each channel in a planar light circuit. For example, in wavelength division multiplexed (WDM) networks, a large number of channels, each with a different wavelength, may be multiplexed together. It is important to know the power of each channel since each channel may be ultimately separated, at its intended destination, from the multiplexed signal.
WDM utilizes a system comprising a plurality of parallel transmitter-receiver pairs. Each of the information sources modulates one of the optical transmitters, each of which produces light at a different wavelength. The modulation bandwidth of each source is narrower than the separation between the wavelengths, resulting in a spectra of the modulated signals which do not overlap. The signals produced by the transmitters are combined into one optical fiber in a WDM multiplexer, which is an optical, and often passive component. At an opposite end of the optical fiber, a WDM demultiplexer, also an optical and often passive component separates the different spectral components of the combined signal from each other. Each of these separated signals is detected by a different receiver. Thus, each signal is assigned a narrow wavelength window in a specific wavelength range.
An arrayed waveguide grating (AWG) is a component used in fiber optics systems employing WDM. The various elements of an AWG are normally integrated onto a single substrate. An AWG comprises a plurality of optical input/output waveguides on both sides of the substrate, two slab waveguides, and a grating that consists of channel waveguides that connect the slab waveguides together, which in turn, connect the input/output guides to the separate channel waveguides. The slab waveguides restrict the propagation of light to the plane perpendicular to the substrate but allow light propagation to both sides of the component. The channel waveguides, on the other hand, prevent light propagation to the sides. The channel waveguides are arranged on a circular arc so that each of them is directed towards a center waveguide of the channel waveguide group or grating on an opposite side of the component.
A constant optical path difference exists between two adjacent channel waveguides in the grating. This path difference is a multiple of the center wavelength used. If light is input from the center input/output waveguide of one side at the center wavelength of the component, the light is distributed to all the waveguides of the grating. As the difference in length of the waveguides is a multiple of the center wavelength, all the waves are in the same phase upon arriving in the output slab waveguide whereupon the light is focused to the center output waveguide. Hence, an AWG focuses different wavelengths to different outputs and the dimensioning of the component determines which wavelengths are focused on which output. Thus, an AWG thus comprises a number of light channels with both focusing characteristics (i.e., a lens) and dispersing characteristics (i.e., the wavelength dependency of the grating).
In an optical communications system, it is often required to adjust the intensity or optical power of the light signals being transmitted. For example, the quality of a signal is determined by the ratio between the intensity of an optical signal and the intensity of noise in the optical signal. This ratio is commonly referred to as the optical signal-to-noise ratio (optical SNR). Therefore, it is often necessary to adjust the intensity of a light signal to increase the optical SNR above a predetermined level.
Moreover, to increase the optical SNR of a wavelength division multiplexed (WDM) signal in an optical communication system, the individual light signals normally must have the same light intensity. However, the intensity of each light signal undesirably varies according to a variation in the output power of the light source generating the light signal and according to variations in the insertion loss of optical components in the optical communication system. Also, an optical amplifier typically has a wavelength dependent gain, which thereby causes the various light signals to have different intensities.
To alleviate this problem, variable optical attenuators (VOA) are typically used to control the intensity of each light signal, and thereby maintain each light signal at the same intensity. Generally, a VOA attenuates, or reduces, the intensity of some of the light signals so that all of the light signals are maintained at the same intensity.
An evanescent coupler is formed with two waveguides disposed together in a substrate and that extend for a coupling distance close to each other, such that the light wave modes passing along each waveguide overlap. The overlap causes some light from one waveguide to pass to the other, and vice versa. The two waveguides in the evanescent coupler separate away from each other outside of the coupling distance.
In the architectures of many photonics devices, such as AWGs, VOAs, optical power monitors, and evanescent couplers, it is desirable to perform optical detection at an upper surface of the planar light circuit (PLC). Accordingly, better ways to detect optical power are needed.